1. Field of the Invention
The present invention relates to an optical disk substrate and a molding material therefor. More specifically, it relates to an optical disk substrate that is formed of an aromatic polycarbonate resin and can maintain high reliability for a long period of time, and a molding material therefor.
2. Prior Art
For a transparent substrate of an optical information recording medium that is a recording medium for recording and/or reproducing information with a laser beam, such as an audio disk, a laser disk, an optical disk memory, a magneto-optical disk, that is, for an optical disk substrate, generally, a polycarbonate resin is used, which is excellent over other resins in moldability, mechanical strength, transparency, and the like. However, the polycarbonate resin having the above excellent properties has a defect that it is easily hydrolyzed at a high temperature under a high humidity to decrease its molecular weight and impact strength. Further, it has a defect that its reliability that should extend for a long period of time is impaired, since a substrate made thereof is caused to have white spots when left at high temperatures under high humidity for a long period of time. At present, further, in substrate materials for high-density optical disks typified by DVD-ROM, DVD-Video, DVD-Audio, DVD-R and DVD-RAM as digital versatile disks (DVD), it is being required to satisfy higher-degree longer-term reliability.
As a method for producing a polycarbonate resin, there is known an interfacial polycondensation method in which a dihydroxy compound and phosgene are directly reacted or a melt-polymerization method in which a dihydroxy compound and carbonate diester are allowed to undergo an ester interchange reaction under heat and under reduced pressure. Of these methods, the later melt-polymerization method has an advantage that a polycarbonate resin can be produced at a low cost as compared with the former interfacial polycondensation method.
Generally, a method for producing an aromatic polycarbonate according to a conventional melt-polymerization method uses, as a catalyst component, a metal catalyst such as an alkali metal compound or an alkaline earth metal compound. For example, JP-A-8-59975 includes a description concerning a method for producing an aromatic polycarbonate according to the melt-polymerization method.
Disk substrates for an optical disk, a laser disk, etc., are generally produced by injection molding, and a molding temperature is a high temperature of 300xc2x0 C. or higher. Further, a continuous production is required, so that the polycarbonate resin is required to have high thermal stability. However, an aromatic polycarbonate resin obtained by a melting method in the presence of the above metal catalyst is sometimes partially pyrolyzed during melt-molding due to a residual metal catalyst, and the aromatic polycarbonate resin is poor in thermal stability. Further, a disk is caused to have white spots in its substrate when left at high temperatures under high humidity for a long period of time, and it has a defect that its reliability that should extend for a long period of time is impaired. In recent years, the disk substrates are increasingly required to have further improved performances including a solution to the above problem.
Meanwhile, for applying an aromatic polycarbonate resin to an optical disk substrate, it is proposed to decrease a gelled substance content in the resin to a specific range.
That is, it is described in JP-A-2-135222 that a gelled substance is present in an aromatic polycarbonate resin and that the content of the gelled substance is decreased to a specific range. In the above known technique, the gelled substance present in the resin causes a refractive index anomaly in an optical use (particularly, a use for an optical disk), so that the number of gelled substances is limited to 50 pieces or less per kg of the resin. The above gelled substance refers to a substance that remains on a filter having openings having a diameter of 20 xcexcm each when a solution of the resin in methylene chloride is filtered. The above known technique is specifically intended for application to a resin obtained by a method in which an aromatic dihydroxy compound and phosgene are reacted in an organic solvent such as methylene chloride (generally referred to as xe2x80x9csolution polymerization methodxe2x80x9d). That is, a resin according to the above solution polymerization method is obtained in the form of a powder, and when the powder is pelletized by extrusion with an extruder, the resin suffers a heat hysteresis in the extruder. The above known technique is intended for limiting the amount of gelled substances that occur during such an occasion to a specific range.
According to studies made by the present inventors, it has been found that, when a resin powder obtained by the above solution polymerization method is melt-pelletized and when pellets are molded into a disk substrate, the number of refractive index anomalies of the disk substrate to be formed is decreased by decreasing the number of gelled substances in the pellets. The present inventors have made further studies and have found that a disk substrate whose gelled substance content is decreased by the above known method shows a decrease in the number of refractive index anomalies immediately after its molding, but that when it is held for a long period of time, particularly, when it is held under high humidity at a high temperature for a long period of time, white spots occur in the substrate and impede reading and reproducing the recorded information. While the cause therefor is not clear, it is presumably caused by inherent impurities such as a catalyst (e.g., sodium, etc.) and an organic solvent (e.g., methylene chloride) used in the solution polymerization method and oligomers.
Meanwhile, the present inventors have studied application of an aromatic polycarbonate resin obtained by a reaction between an aromatic dihydroxy compound and a carbonate diester (also generally called xe2x80x9cmelt polymerization methodxe2x80x9d) to disk substrates.
In the above melt polymerization method, a resin suffers heat hysteresis at a high temperature for a long period of time beyond comparison in the process of the polymerization as compared with the above solution polymerization. Therefore, there occur a large amount of undissolved substances that are insoluble in methylene chloride solvent while they are not removable through a filter of an extruder. The present inventors have studied these undissolved substances and have found that the behavior thereof differs from that of the above gelled substance involved in the solution polymerization method. The types and numbers of the undissolved substances in a resin obtained by a melt polymerization method are larger than the types and number of those in a resin obtained by a solution polymerization method. Studies have been made with regard to influences of the types and number of the undissolved substances on refractive index anomalies and the formation of white spots found after holding for a long period of time.
As a result, it has been found that the number of luminous undissolved substances generated by irradiation with specific wavelength (wavelength of 380 nm), of the undissolved substances in a resin, has something to do with the number of white spots that occur after the holding for a long period of time and that the number of white spots to occur can be decreased to a tolerance limit or less by decreasing such specific undissolved substances to a specific number or less.
That is, according to the studies by the present inventors, the tolerance range of the number of the undissolved substances that emit light by irradiation with a wavelength of 380 nm, in a resin obtained by a melt polymerization method, is 100 pieces or less per kg of the resin. While this tolerance range is broader than the tolerance range (50 pieces or less) of gelled substances in the above known technique, it is presumably because the behavior of the undissolved substances caused by the inherent catalyst and polymerization conditions of the melt polymerization method differs from the counterpart in the solution polymerization method that the number of the white spots to occur after a disk substrate is held for a long period of time is remarkably decreased.
The present invention has been arrived at on the basis of the revealed fact above.
The undissolved substances that emit light by irradiation with light having a wavelength of 380 nm in the polycarbonate resin is assumed to be substances that emit fluorescence due to salicylic acid ester structure thereof. In an optical disk substrate formed of a polycarbonate resin produced by a melt-polycondensation method using a catalyst in a high-temperature reduced pressure state, the content of the above substances tends to increase. However, it has not been known that an optical disk substrate is improved in reliability for a long period of time by decreasing the above light-emitting substances to a specific range or less.
According to the present invention, there is provided a molding material for optical use, which is an aromatic polycarbonate resin obtained by a reaction between an aromatic dihydroxy compound and a carbonate diester and in which the content of undissolved substances that emit light by irradiation with light having a wavelength of 380 nm and have a size of 30 xcexcm or greater is 100 pieces or less per kg of said resin.
According to the present invention, further, there is provided an optical disk substrate formed of an aromatic polycarbonate resin obtained by a reaction between an aromatic dihydroxy compound and a carbonate diester and in which the content of undissolved substances that emit light by irradiation with light having a wavelength of 380 nm and have a size of 30 xcexcm or greater is 100 pieces or less per kg of said resin.
The present invention will be explained more specifically hereinafter.
The polycarbonate resin used in the present invention is a resin obtained by a melt-polymerization method based on an ester interchange of a dihydric phenol and a carbonate precursor. Typical examples of the dihydric phenol used above include hydroquinone, resorcinol, 1,6-dihydroxynaphthalene, 2,6-dihydroxynaphthalene, 4,4xe2x80x2-dihydroxydiphenyl, bis(4-hydroxyphenyl)methane, 1,1-bis(4-hydroxyphenyl)-1-phenylmethane, bis{(4-hydroxy-3,5-dimethyl)phenyl}methane, 1,1-bis(4-hydroxyphenyl)ethane, 1,2-bis(4-hydroxyphenyl)ethane, 1,1-bis(4-hydroxyphenyl)-1-phenylethane, bis(4-hydroxyphenyl)diphenylmethane, bis(4-hydroxyphenyl)-1-naphthylmethane, 2,2-bis(4-hydroxyphenyl)propane (so-called bisphenol A), 2-(4-hydroxyphenyl)-2-(3-hydroxyphenyl)propane, 2,2-bis{(4-hydroxy-3-methyl)phenyl}propane, 2,2-bis{(4-hydroxy-3,5-dimethyl)phenyl}propane, 2,2-bis{(3,5-dibromo-4-hydroxy)phenyl}propane, 2,2-bis{(3,5-dichloro-4-hydroxy)phenyl}propane, 2,2-bis{(3-bromo-4-hydroxy)phenyl}propane, 2,2-bis{(3-chloro-4-hydroxy)phenyl}propane, 4-bromoresorcinol, 2,2-bis{(3-isopropyl-4-hydroxy)phenyl}propane, 2,2-bis{(3-phenyl-4-hydroxy)phenyl}propane, 2,2-bis{(3-ethyl-4-hydroxy)phenyl}propane, 2,2-bis{(3-n-propyl-4-hydroxy)phenyl}propane, 2,2-bis{(3-sec-butyl-4-hydroxy)phenyl}propane, 2,2-bis{(3-tert-butyl-4-hydroxy)phenyl}propane, 2,2-bis{(3-cyclohexyl-4-hydroxy)phenyl}propane, 2,2-bis{(3-methoxy-4-hydroxy)phenyl}propane, 2,2-bis(4-hydroxyphenyl)hexafluoropropane, 1,1-dibromo-2,2-bis(4-hydroxyphenyl)ethylene, 1,1-dichloro-2,2-bis{(3-phenoxy-4-hydroxy)phenyl}ethylene, ethylene glycol bis(4-hydroxyphenyl)ether, 2,2-bis(4-hydroxyphenyl)butane, 2,2-bis(4-hydroxyphenyl)-3-methylbutane, 2,2-bis(4-hydroxyphenyl)-3,3-dimethylbutane, 2,4-bis(4-hydroxyphenyl)-2-methylbutane, 1,1-bis(4-hydroxyphenyl)isobutene, 2,2-bis(4-hydroxyphenyl)pentane, 2,2-bis(4-hydroxyphenyl)-4-methylpentane, 3,3-bis(4-hydroxyphenyl)pentane, 1,1-bis(4-hydroxyphenyl)cyclohexane, 1,1-bis(4-hydroxyphenyl)-4-isopropylcyclohexane, 1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane, 1,1-bis(4-hydroxyphenyl)cyclodecane, 9,9-bis(4-hydroxyphenyl)fluorene, 9,9-bis{(4-hydroxy-3-methyl)phenyl}fluorene, xcex1,xcex1xe2x80x2-bis(4-hydroxyphenyl)-o-diisopropylbenzene, xcex1,xcex1xe2x80x2-bis(4-hydroxyphenyl)-m-diisopropylbenzene, xcex1,xcex1xe2x80x2-bis(4-hydroxyphenyl)-p-diisopropylbenzene, 1,3-bis(4-hydroxyphenyl)-5,7-dimethyladamantane, 4,4xe2x80x2-dihydroxydiphenylsulfone, bis{(3,5-dimethyl-4-hydroxy)phenyl}sulfone, 4,4xe2x80x2-dihydroxydiphenylsulfoxide, 4,4xe2x80x2-dihydroxydiphenylsulfide, 4,4xe2x80x2-dihydroxydiphenylketone, 4,4xe2x80x2-dihydroxydiphenyl ether and 4,4xe2x80x2-dihydroxydiphenyl ester. These may be used alone or in combination of two or more.
Our of these, preferred is a homopolymer or copolymer obtained from at least one bisphenol selected from the group consisting of bisphenol A, 2,2-bis{(4-hydroxy-3-methyl)phenyl}propane, 2,2-bis{(3,5-dibromo-4-hydroxy)phenyl}propane, ethylene glycol bis(4-hydroxyphenyl)ether, 2,2-bis(4-hydroxyphenyl)hexafluoropropane, 2,2-bis(4-hydroxyphenyl)butane, 1,1-bis(4-hydroxyphenyl)cyclohexane, 1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane, 4,4xe2x80x2-dihydroxydiphenylsulfone, bis{(3,5-dimethyl-4-hydroxy)phenyl}sulfone, 4,4xe2x80x2-dihydroxydiphenylsulfoxide, 4,4xe2x80x2-dihydroxydiphenylsulfide, and 4,4xe2x80x2-dihydroxydiphenyl ketone. A homopolymer of bisphenol A is particularly preferred.
The carbonate precursor is selected from carbonate ester or haloformate. Specifically, the carbonate precursor includes diphenyl carbonate, ditolyl carbonate, bis(chlorophenyl)carbonate, m-cresyl carbonate, dinaphthyl carbonate, bis(diphenyl)carbonate, dimethyl carbonate, diethyl carbonate, dibutyl carbonate and dicyclohexyl carbonate, although the carbonate precursor shall not be limited thereto. Preferably, diphenyl carbonate or dihaloformate of a dihydric phenol is used, and more preferably, diphenyl carbonate is used. These carbonate esters may be used alone or in combination of two or more.
When the polycarbonate resin is produced by reacting the above dihydric phenol and the above carbonate precursor according to a melt-polymerization method, a catalyst, a terminal stopper and an antioxidant for the dihydric phenol may be used as required. The polycarbonate resin may be a polyester carbonate resin formed by copolymerizing an aromatic or aliphatic difunctional carboxylic acid or may be a mixture containing two or more polycarbonate resins obtained.
The reaction according to a melt-polymerization method is an ester interchange reaction between the dihydric phenol and the carbonate ester, and the reaction is carried out by a method in which in the presence of an inert gas, the dihydric phenol and the carbonate ester are mixed under heat and a formed alcohol or phenol is distilled off. While the reaction temperature differs depending upon the boiling point, etc., of the formed alcohol or phenol, it is generally in the range of from 120 to 350xc2x0 C. In a later stage of the reaction, the formed alcohol or phenol can be easily distilled off by reducing the pressure of the reaction system to approximately 10 to 0.1 Torr (1,333 to 13.3 MPa). The reaction time period is generally about 1 to 4 hours.
In the melt-polymerization method, further, a polymerization catalyst may be used for promoting the polymerization rate. As a polymerization catalyst, for example, a catalyst containing (i) an alkali metal compound and/or (ii) a nitrogen-containing basic compound is used, and condensation is carried out.
Examples of the alkali metal compound used as a catalyst include hydroxide, hydrogencarbonate, carbonate, acetate, nitrate, nitrite, sulfite, cyanate, thiocyanate, stearate, hydroborate, benzoate and phosphorohydride of alkali metal, and alkali metal salts of bisphenol and phenol.
Specific examples of the alkali metal compound include sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium hydrogencarbonate, potassium hydrogencarbonate, lithium hydrogencarbonate, sodium carbonate, potassium carbonate, lithium carbonate, sodium acetate, potassium acetate, lithium acetate, sodium nitrate, potassium nitrate, lithium nitrate, sodium nitrite, potassium nitrite, lithium nitrite, sodium sulfite, potassium sulfite, lithium sulfite, sodium cyanate, potassium cyanate, lithium cyanate, sodium thiocyanate, potassium thiocyanate, lithium thiocyanate, sodium stearate, potassium stearate, lithium stearate, sodium boron hydroxide, lithium boronhydroxide, potassium boronhydride, sodium boron phenylate, sodium benzoate, potassium benzoate, lithium benzoate, disodium hydrogen phosphate, dipotassium hydrogen phosphate, dilithium hydrogen phosphate, disodium salt, dipotassium salt and dilithium salt of bisphenol A, and sodium salt, potassium salt and lithium salt of phenol.
The alkali metal compound as a catalyst can be used in an amount range of from 10xe2x88x929 to 10xe2x88x924 mol, preferably 10xe2x88x928 to 10xe2x88x925 mol, per mole of the dihydric phenol. When the amount of the alkali metal compound is outside the above range, undesirably, there is a problem that it causes a detrimental effect on various physical properties of a polycarbonate to be obtained, or that the ester interchange reaction does not fully proceed, so that a polycarbonate having a high molecular weight cannot be obtained.
Examples of the nitrogen-containing basic compound as a catalyst include ammonium hydroxides having an alkyl, aryl or alkylaryl group such as tetramethylammonium hydroxide (Me4NOH), tetraethylammonium hydroxide (Et4NOH), tetrabutylammonium hydroxide (Bu4NOH), benzyltrimethylammonium hydroxide (xcfx86-CH2(Me)3NOH) and hexadecyltrimethylammonium hydroxide; tertiary amines such as triethylamine, tributylamine, dimethylbenzylamine, and hexadecyldimethylamine; and basic salts such as tetramethylammonium borohydride (Me4NBH4), tetrabutylammonium borohydride (Bu4NBH4), tetrabutylammonium tetraphenylborate (Bu4NBPh4) and tetramethylammonium tetraphenylborate, (Me4NBPh4). Of these, tetramethylammonium hydroxide (Me4NOH), tetraethylammonium hydroxide (Et4NOH) and tetrabutylammonium hydroxide (Bu4NOH) are preferred, and tetramethylammonium hydroxide (Me4NOH) is particularly preferred.
The above nitrogen-containing basic compound is preferably used in such an amount that the amount of ammonium nitrogen atoms in the nitrogen-containing basic compound per mole of the dihydric phenol is from 1xc3x9710xe2x88x925 to 1xc3x9710xe2x88x923 equivalent weight. The above amount is more preferably such that the amount based on the same standard is from 2xc3x9710xe2x88x925 to 7xc3x9710xe2x88x924 equivalent weight, and particularly preferably such that the amount based on the same standard is from 5xc3x9710xe2x88x925 to 5xc3x9710xe2x88x924 equivalent weight.
In the present invention, there may be used a catalyst generally used for an esterification or ester interchange reaction as required, and such catalyst includes alkoxides of an alkali metal or alkaline earth metal, organic acid salts of an alkali metal or alkaline earth metal, zinc compounds, boron compounds, aluminum compounds, silicon compounds, germanium compounds, organotin compounds, lead compounds, osmium compounds, antimony compounds, manganese compounds, titanium compounds and zirconium compounds. The above catalysts may be used alone or in combination of two or more. The amount of the above polymerization catalyst per mole of the dihydric phenol as a raw material is determined preferably to be 1xc3x9710xe2x88x929 to 1xc3x9710xe2x88x925 equivalent weight, more preferably to be 1xc3x9710xe2x88x928 to 5xc3x9710xe2x88x926 equivalent weight.
In the above polymerization, further, the following compound may be added at a later stage, or after the end of the polycondensation, for decreasing phenolic terminal groups. Such a compound includes phenol, p-tert-butylphenol, p-tert-butylphenylphenyl carbonate, p-tert-butylphenyl carbonate, p-cumylphenol, p-cumylphenylphenyl carbonate, p-cumylphenyl carbonate, bis(chlorophenyl)carbonate, bis(bromophenyl)carbonate, bis(nitrophenyl)carbonate, bis(phenylphenyl)carbonate, chlorophenylphenyl carbonate, bromophenylphenyl carbonate, nitrophenylphenyl carbonate, diphenyl carbonate, methoxycarbonylphenylphenyl carbonate, 2,2,4-trimethyl-4-(4-hydroxyphenyl)chroman, 2,4,4-trimethyl-2-(4-hydroxyphenyl)chroman and ethoxycarbonylphenylphenyl carbonate. Of these, 2-chlorophenylphenyl carbonate, 2-methoxycarbonylphenylphenyl carbonate and 2-ethoxycarbonylphenylphenyl carbonate are preferred, and 2-methoxycarbonylphenylphenyl carbonate is particularly preferred.
In the present invention, terminals of the polycarbonate resin may be blocked with a terminal blocker. Further, desirably, the concentration of terminal hydroxyl groups of the polycarbonate resin before addition of the terminal blocker on the basis of the total terminals is adjusted to at least 20 mol %, preferably to at least 30 mol %, still more preferably to at least 40 mol %. In this manner, a specific terminal group can be introduced at a high amount ratio, and a high modification effect of the polycarbonate resin can be attained. Generally, concerning the concentration of terminal hydroxy groups of the polycarbonate resin based on the total terminals, it is advantageous to use the terminal blocker for a polycarbonate resin having hydroxyl groups whose concentration based on the total terminals is in the range of from 30 to 95 mol %. The amount ratio of the hydroxyl group terminals of the polycarbonate resin before addition of the terminal blocker can be controlled on the basis of the amount ratio of the dihydric phenol and the diphenyl carbonate that are charged as raw materials. The molar amount of the above concentration of the terminal hydroxyl groups in a constant amount of the polycarbonate resin can be determined by a conventional method using 1H-NMR.
When the molar amount of the total terminals of the polycarbonate resin of the present invention is 100 mol %, the molar amount of the terminal hydroxyl group of the polycarbonate resin is controlled to be preferably 10 to 70 mol %, more preferably 15 to 65 mol %, still more preferably 20 to 60 mol %, most preferably 20 to 45 mol %. The above mole percentage of the terminal hydroxyl groups of the aromatic polycarbonate resin can be determined by a conventional method using 1H-NMR.
The molecular weight, as a viscosity average molecular weight (M), of the polycarbonate resin is preferably 10,000 to 22,000, more preferably 12,000 to 20,000, particularly preferably 13,000 to 19,000. The aromatic polycarbonate resin having such a viscosity average molecular weight is preferred, since it gives sufficient strength, attains excellent melt flowabillty during molding and causes no molding strain. The viscosity average molecular weight that is referred to in the present invention is determined by inserting into the following expression a specific viscosity (xcex7sp) determined using a solution prepared by dissolving 0.7 g of the polycarbonate resin in 100 ml of methylene chloride at 20xc2x0 C.
xcex7sp/c=[xcex7]+0.45xc3x97[xcex7]2c (in which [xcex7] is an intrinsic viscosity)
[xcex7]=1.23xc3x9710xe2x88x924M0.83 
c=0.7
After the polycarbonate resin is produced by a melt-polymerization method known per se, and in an extrusion step of obtaining a polycarbonate resin in the form of pellets to be supplied to injection molding (pelletization step), preferably, foreign matter is removed through a sintered metal filter having a filtering accuracy of 10 xcexcm when the polycarbonate resin is in a molten state. It is preferred to add additives such as phosphorus-based antioxidant, etc., as required. In any case, it is required to decrease the contents of foreign matter, impurities, a solvent, etc., in the resin as a raw material before the injection molding so as to make them as small as possible. When an optical disk substrate is produced from the above polycarbonate resin, an injection molding machine (including an injection compression molding machine) is used. While the above injection molding machine can be selected from generally used injection molding machines, it is preferred to use an injection molding machine having a cylinder and a screw made of a material that has low adhesion to the resin and exhibits anti-corrosion properties and anti-wearing properties, in view of prevention of occurrence of a carbonaceious material and an improvement in reliability of the disk substrate. Concerning injection molding conditions, preferably, the cylinder temperature is from 300 to 400xc2x0 C. and the mold temperature is from 50 to 140xc2x0 C., and under these conditions, an optically excellent optical disk substrate can be obtained. In view of the object of the present invention, preferably, the molding environment is as clean as possible. It is also important to remove water by fully drying the material that is to be supplied to the molding and take care not to cause a residence that may cause decomposition of a molten resin.
The optical disk substrate of the present invention is formed of an aromatic polycarbonate resin which is obtained by a melt-polymerization method and in which the content of undissolved substances that emit light by irradiation with light having a wavelength of 380 nm and have a size of 30 xcexcm or greater is 100 pieces or less per kg of the resin.
The undissolved substances that emit light by irradiation with light having a wavelength of 380 nm will be sometimes abbreviated as xe2x80x9clight-emitting undissolved substancesxe2x80x9d hereinafter.
While the measurement of the above light-emitting undissolved substances will be explained in detail later, the measurement is conducted by dissolving a polycarbonate resin in methylene chloride, filtering a solution through a filter having openings having a diameter of 30 xcexcm each (opening diameter), drying a residue on the filter and counting the number of substances that emit light by irradiation with light having a wavelength of 380 nm while observing the residue through an optical microscope. The number of the substances that emit light is converted to a value per kg of the resin, and the value. is taken as the content of the light-emitting undissolved substances.
The optical disk substrate of the present invention is formed of a polycarbonate resin having a light-emitting undissolved substance content of 100 pieces or less. The content of the light-emitting undissolved substances in the resin is preferably 80 pieces or less, particularly preferably 50 pieces or less.
In the present invention, it has been found that any optical disk substrate formed of a polycarbonate resin whose light-emitting undissolved substance content is decreased to the specific range or less as described above shows remarkably decreased occurrences of white spots not only immediately after molding but also after the passage of a long period of time. The optical disk substrate of the present invention is therefore excellent in storage of recordings and stability for a long period of time.
The present invention accordingly uses the resin whose light-emitting undissolved substance content is decreased to the above range, so that there can be provided optical disk substrates in which white spots having a size of 20 xcexcm or greater each occur at an average of two or less per disk substrate (disk) having a diameter of 120 mm in an accelerated deterioration test of holding the optical disk substrates under conditions of a temperature of 80xc2x0 C. and a relative humidity of 85% for 1,000 hours. Under optimum conditions, there are provided optical disk substrates in which the number of occurrence of the white spots is an average of 1.5 pieces or less, and under particularly optimum conditions, there are provided optical disk substrates in which the number of occurrence of the white spots is an average of 1 piece or less.
In the present invention, the means for obtaining a polycarbonate resin having a light-emitting undissolved substance content satisfying the above specific range includes the following means.
(1) Method in which a polycarbonate resin is dissolved in a good solvent such as methylene chloride, and a solution is filtered through a filter having openings having a diameter of 30 xcexcm (opening diameter) or less at a normal temperature under normal pressure, to remove solids.
(2) When a polymerization catalyst, particularly, the sodium metal compound, is used as a catalyst in the polymerization of the polycarbonate resin, the basic nitrogen-containing compound is used in combination so that the amount of the sodium metal compound per mole of the aromatic dihydroxy compound is decreased to 1xc3x9710xe2x88x928 to 1xc3x9710xe2x88x925 mol, preferably 1xc3x9710xe2x88x928 to 5xc3x9710xe2x88x926 mol, particularly preferably 1xc3x9710xe2x88x928 to 6xc3x9710xe2x88x927 mol.
(3) In the step of producing the polycarbonate, polymerization conditions, particularly, temperature conditions are controlled. That is, means are selected such that the temperature in a highest temperature zone in the polymerization step does not exceed 340xc2x0 C. Specifically, in the polymerization step, the number of rotation of a stirring blade is controlled. Further, means are selected such that a polymer temperature difference between a low-temperature zone and a high-temperature zone during the polymerization step (in a polymerization reactor) does not exceed 50xc2x0 C. Such means will be explained further specifically later.
(4) The content of a polyfunctional compound in the dihydric phenol as a raw material, particularly in bisphenol, is decreased. That is, if the dihydric phenol as a raw material contains, as impurities, trifunctional or higher-functional compounds such as triphenol and tetraphenol, part of them cause light-emitting undissolved substances to occur.
Examples of the above triphenol includes compounds of the following formulae (I) and (II). 
(5) The carbonate diester as a raw material is selected from those whose sodium compound content is very small. The carbonate diester, particularly diphenyl carbonate, contains a very small amount of a sodium compound due to a catalyst used in the step of production thereof. Since the above sodium compound contained in a very small amount has not a little influence on the occurrence of the light-emitting undissolved substances in the polymerization, the diphenyl carbonate as a raw material is selected from those in which the total content of the sodium compound is very small.
Of the above (1) to (5), (2) to (5) are means of preventing the occurrence of the light-emitting undissolved substances, and it is desirable to employ a proper combination thereof. Naturally, these means (1) to (5) are mere illustrative examples, and other means may be employed. Further, any combination of these means may be employed, or these means may be used in combination with other means.
According to studies made by the present inventors, it has been found that there can be obtained a disk substrate that is further improved in thermal stability for a long period of time and exhibits a decreased number of white spots occurring, by decreasing the content of the light-emitting undissolved substances in the polycarbonate resin to the above specific range and further by (i) adjusting the relative fluorescence intensity of the resin to a specific value or less and/or (ii) adjusting the activity index of a residual catalyst of the resin to a specific value or less.
According to the present invention, further, there are provided the following disk substrates (I) to (III).
(I) An optical disk substrate formed of a resin
(A) that is an aromatic polycarbonate resin obtained by a reaction between an aromatic dihydroxy compound and a carbonate diester,
wherein:
(B) the content of undissolved substances that emit light by irradiation with light having a wavelength of 380 nm and have a size of 30 xcexcm or greater is 100 pieces or less per kg of said resin, and
(C) the resin has a relative fluorescence intensity, based on a reference substance, of 4xc3x9710xe2x88x923 or less at 465 nm when the resin is measured for fluorescence spectrum.
(II) An optical disk substrate formed of a resin
(A) that is an aromatic polycarbonate resin obtained by a reaction between an aromatic dihydroxy compound and a carbonate diester,
wherein:
(B) the content of undissolved substances that emit light by irradiation with light having a wavelength of 380 nm and have a size of 30 xcexcm or greater is 100 pieces or less per kg of said resin, and
(D) the resin has a residual catalyst activity index of 2% or less.
(III) An optical disk substrate formed of a resin
(A) that is an aromatic polycarbonate resin obtained by a reaction between an aromatic dihydroxy compound and a carbonate diester,
wherein:
(B) the content of undissolved substances that emit light by irradiation with light having a wavelength of 380 nm and have a size of 30 xcexcm or greater is 100 pieces or less per kg of said resin,
(C) the resin has a relative fluorescence intensity, based on a reference substance, of 4xc3x9710xe2x88x923 or less at 465 nm when the resin is measured for fluorescence spectrum, and
(D) the resin has a residual catalyst activity index of 2% or less.
In each of the above disk substrates (I) to (III) after the accelerated deterioration test (80xc2x0 C.xc3x9785%RHxc3x971,000 hours), desirably, the number of white spots having a size of 20 xcexcm or greater per disk substrate having a diameter of 120 mm is an average of 2 pieces or less, preferably an average of 1.5 pieces or less.
When the above disk substrates (I) to (III) is measured for fluorescence spectrum, the relative fluorescence intensity of the resin based on a reference substance at 465 nm is 4xc3x9710xe2x88x923 or less, preferably 3xc3x9710xe2x88x923 or less, particularly preferably 2xc3x9710xe2x88x923 or less. When the above relative intensity exceeds the above value, the substrate tends to show a decrease in humidity resistance, heat resistance and mechanical properties.
Desirably, the following means is employed for obtaining a polycarbonate resin having a relative fluorescence intensity of the above specific value or less. Preferably, the amount of the polymerization catalyst is to be defined as described above, said polymerization catalyst is to be deactivated with a sulfonic acid compound, and the amount ratio of hydroxy groups to all the molecular terminals is to be defined with regard to terminals of molecule of the polycarbonate.
Further, preferably, the temperature of the polycarbonate resin in the melt-polymerization reaction is constantly maintained at 300xc2x0 C. or lower, particularly preferably at 255xc2x0 C. or lower, for obtaining a polycarbonate resin having a relative fluorescence intensity of the above specific value or less.
Further, with regard to stirring with a polymerizer stirring blade, it is preferred to adjust a value obtained by dividing the stirring shear rate of the polymerizer stirring blade (unit: 1/sec) represented by the following equation by a square of radius of the stirring blade (unit: cm) to 0.1 to 0.001 (l/(secxc3x97cm2)), for obtaining a polycarbonate resin having a relative fluorescence intensity of the above specific value or less.
Stirring shear rate=peripheral velocity of stirring blades/length of gap between reactor and stirring blade (in which the unit of the stirring shear rate is l/sec, the unit of the peripheral velocity of the stirring blade is cm/sec, and the length of the gap of the stirring blade is cm).
With regard to the catalyst system in the production of the polycarbonate resin, a basic nitrogen compound and an alkali metal compound (particularly, a sodium compound) are used, and in this case, the amount of the alkali metal compound is controlled to be 5.0xc3x9710xe2x88x926 mol or less per mole of the dihydric phenol, whereby a polycarbonate having a relative fluorescence intensity of a low value can be obtained. It is preferred to employ the above means in a proper combination.
In the disk substrates (II) and (III), essentially, the resin has a residual catalyst activity index of 2% or less, preferably 1% or less.
In the polycarbonate resin obtained by melt-polymerization, a polymerization catalyst is used for promoting the reaction thereof, and the polymerization catalyst often remains after the polymerization. If the remaining catalyst is left as it is after completion of the polymerization, there is caused a detrimental effect that the catalytic activity of the polymerization catalyst causes the polycarbonate resin to undergo decomposition or a re-reaction. Further, in the polycarbonate resin having such residual catalyst activity, not only the influence thereof spreads, but also there is sometimes caused a new problem on the retention of performances of the disk substrate, so that it is preferred to inhibit the residual catalyst activity.
Measurement is carried out in the following manner using a residual catalyst activity index as an index for inhibiting the residual catalyst activity. A rotary rheometer that can measure a sample as a measurement object for a value in a melt viscosity range is used as a measuring device, and a change in melt viscosity is observed while the sample is rotated in a constant direction at a constant angular velocity in a nitrogen current sufficient for the freedom of the sample from oxidation with external oxygen under constant-temperature conditions where a resin to be measured is melted. As a tool for a viscoelasticity measuring device for measuring the sample, a tool having the form of a conical disk is used such that a strain in the entire sample is constant, that is, that the shear speed comes to be constant. That is, a change in melt viscosity per minute, calculated on the basis of the following expression (i), is taken as a residual catalyst activity index.                               Residual          ⁢                      xe2x80x83                    ⁢          catalyst          ⁢                      xe2x80x83                    ⁢          activity          ⁢                      xe2x80x83                    ⁢          index          ⁢                      xe2x80x83                    ⁢                      (            %            )                          =                                                                                                  "LeftBracketingBar"                                          (                                                                        Melt                          ⁢                                                      xe2x80x83                                                    ⁢                          viscosity                          ⁢                                                      xe2x80x83                                                    ⁢                          after                          ⁢                                                      xe2x80x83                                                    ⁢                          30                          ⁢                                                      xe2x80x83                                                    ⁢                          minutes                                                -                                                                                                                                                                                                                                      melt                          ⁢                                                      xe2x80x83                                                    ⁢                          viscosity                          ⁢                                                      xe2x80x83                                                    ⁢                          after                          ⁢                                                      xe2x80x83                                                    ⁢                          5                          ⁢                                                      xe2x80x83                                                    ⁢                          minutes                                                )                                            "RightBracketingBar"                                        ⁢                                          xe2x80x83                                                                                                          Melt              ⁢                              xe2x80x83                            ⁢              viscosity              ⁢                              xe2x80x83                            ⁢              after              ⁢                              xe2x80x83                            ⁢              5              ⁢                              xe2x80x83                            ⁢              minutes              xc3x97              25                                xc3x97          100                                    (        i        )            
The above residual catalyst activity index is preferably 2% or less, more preferably 1% or less, still more preferably 0.5% or less, and most preferably 0.2% or less. When the residual catalyst activity index is in the above range, desirably, there is almost no change in performances of the disk substrate with the passage of time.
For bringing the residual catalyst activity index of the resin into the above value, effectively, not only the amount of the polymerization catalyst is relatively decreased, but also a deactivator for removing the activity of the catalyst is added to the resin after completion of the polymerization. Examples of the above deactivator include bezenesulfonic acid, p-toluenesulfonic acid; sulfonate esters such as methyl benzenesulfonate, ethyl benzenesulfonate, butyl benzenesulfonate, octyl benzenesulfonate, phenyl benzenesulfonate, methyl p-toluenesulfonate, ethyl p-toluenesulfonate, butyl p-toluenesulfonate, octyl p-toluenesulfonate and phenyl p-toluenesulfonate: and further include compounds such as trifluoromethanesulfonic acid, naphthalenesulfonic acid, sulfonated polystyrene, a methyl acrylate-sulfonated styrene copolymer, 2-phenyl-2-propyl dodecylbenzenesulfonate, 2-phenyl-2-butyl dodecylbenzenesulfonate, octylsulfonic acid tetrabutylphosphonium salt, decylsulfonic acid tetrabutylphosphonium salt, benzenesulfonic acid tetrabutylphosphonium salt, dodecylbenzenesulfonic acid tetraethylphosphonium salt, dodecylbenzenesulfonic acid tetrabutylphosphonium salt, dodecylbenzenesulfonic acid tetrahexylphosphonium salt, dodecylbenzenesulfonic acid tetraoctylphosphonium salt, decylammonium butylsulfate, decylammonium decylsulfate, dodecylammonium methylsulfate, dodecylammonium ethylsulfate, dodecylmethylammonium methylsulfate, dodecyldimethylammonium tetradecylsulfate, tetradecyldimethylammonium methylsulfate, tetramethylammonium hexylsulfate, decyltrimethylammonium hexadecylsulfate, tetrabutylammonium dodecylbenzylsulfate, tetraethylammonium dodecylbenzylsulfate and tetramethylammonium dodecylbenzylsulfate, although the deactivator shall not be limited thereto. These compounds may be used in combination of two or more.
Of these deactivators, phosphonium or ammonium salt type deactivators are advantageous since they are stable themselves at 200xc2x0 C. or higher. When the deactivator is added to the polycarbonate resin, it promptly neutralizes the polymerization catalyst to give a stable polycarbonate resin. That is, the amount of the deactivator based on the polycarbonate resin formed after the polymerization is preferably 0.01 to 500 ppm, more preferably 0.01 to 300 ppm, particularly preferably 0.01 to 100 ppm.
Concerning the amount ratio of the above deactivator to the polymerization catalyst, further, it is preferred to use the above deactivator in an amount of 0.5 to 50 mol per mole of the polymerization catalyst. The method of adding the deactivator to the polycarbonate resin after the polymerization is not restricted. For example, the deactivator may be added while the polycarbonate resin as a reaction product is in a molten state, or it may be added to the polycarbonate resin that is once pelletized and then re-melted. In the former, while the polycarbonate resin that is a reaction product in a molten state in a reactor or an extruder after completion of the reaction is in a molten state, the deactivator may be added, and the polycarbonate resin may be molded and then pelletized through the extruder. Alternatively, the deactivator may be added and kneaded with the polycarbonate resin at any time when the polycarbonate resin obtained by the polymerization passes from the reactor to the extruder and then pelletized, whereby the polycarbonate resin is obtained.